The present invention relates to a flow meter for measuring a flow rate of fluid.
FIG. 6 illustrates a conventional flow meter. At a conduit 1 in which fluid flows, a first oscillator 2 and a second oscillator 3 which face each other across the fluid for transmitting and receiving an ultrasonic wave are mounted. A driver 4 drives the first oscillator 2 to produce and transmit an ultrasonic wave. The ultrasonic wave is then received by the second oscillator 3 and transferred to a received-wave detector 7 for detecting the timing of receiving the wave. A timer 8 counts a time from the start of actuation of the driver 4 for driving the first oscillator 2 to the detection of the timing by the received-wave detector 7, and determines a propagation duration T1 of the ultrasonic wave. Then, a switching unit switches the direction of the transmitting/receiving of the ultrasonic wave, and the time counter 8 measures a propagation duration T2 of the ultrasonic wave in a reverse direction. A flow-rate calculator 9 then calculates the flow rate of the fluid from durations T1 and T2 in both the directions.
The received-wave detector 7 will be explained in more detail referring to FIG. 7.
The received-wave detector 7 includes a third-peak detector for detecting a third peak W3 from the rise of the waveform, and a zero-cross detector for detecting a zero-cross point of the received wave. Upon receiving the propagated wave, the third-peak detector detects a third peak W3 (denoted by xe2x80x9cAxe2x80x9d in FIG. 7) from the rise of the waveform, and outputs its detection signal for actuating the zero-cross detector. The zero-cross detector then detects a zero-cross point (denoted by B in FIG. 7) succeeding the peak, and determines the timing of receiving the wave. The timing is then transferred to the time counter 8. The timing of receiving the propagated wave determined by the above sequence provides the durations T1 and T2 of the ultrasonic wave. Then, the flow-rate calculator 9 calculating the flow rate Q from the duration T1 and T2 as equation 1,
Q=(T1xe2x88x92T2)/(T1xc3x97T2)xc3x97Kxe2x80x83xe2x80x83(Equation 1)
where K is a constant determined according to a cross sectional area of the conduit, a propagating distance of the ultrasonic wave, the positional relationship between the oscillators and the conduit, and their units.
Another conventional flow meter is disclosed in Japanese Patent Laid-open Publication No. 8-70926. In the meter, a circuit connected to two oscillators is matched in impedance so as to be equal in impedance in both a transmission mode and a reception mode and then connected to a transmitter/receiver circuit. The impedance in the circuit is low and constant. Another conventional flow meter detects the amplitude of a received wave at some points in time synchronized with a reference clock signal, and produces data of a phase against the reference clock signal according to the relationship between the detected amplitude and the timing of the reference clock signal. Then, the propagation duration of the wave can be determined from a combination of rough timings of the reference clock signal and the data of the phase having high resolution. This measuring method may however create an error due to a change of the amplitude of the received wave converted into the data of the timing. The method requires that the waveform of the ultrasonic wave propagated from its upstream to downstream and the waveform of a reverse ultrasonic wave propagated from the downstream to the upstream are shaped identical to each other. Thus, respective impedances between the transmission mode and the reception mode are matched.
As shown in equation 1, a relative accuracy of the durations T1 and T2, i.e., a difference (T1xe2x88x92T2) affects the accuracy of the measurement of the flow rate more than respective absolute value of the durations T1 and T2. For increasing the relative accuracy of the durations T1 and T2, delay times for which the ultrasonic wave is received by the oscillator and detected as a converted electrical signal by at the received-wave detector 7 have to be identical between both the directions of the transmission and reception.
As shown in an equivalent circuit of FIG. 2, a receiving-side oscillator of the conventional flow meter includes a signal source 11 for converting the ultrasonic wave into an electric signal, an internal impedance 12 (Zo), and an inter-electrode capacitance 13 (C). Upon receiving the ultrasonic wave, the oscillator produces an electric signal from the oscillation of the ultrasonic wave with a delay determined by the internal impedance 12 and the inter-electrode capacitance 13. More particularly, the delay is proportional to (Cxc3x97Zo), thus increasing as the internal impedance 12 and the inter-electrode capacitance 13 are increased.
When an ambient temperature varies, a change in the inter-electrode capacitance 13 increases significantly, thus changing the delay of the output signal from the oscillator. This makes measurement of the duartions T1 and T2 inaccurate. The higher an input impedance of the received-wave detector 7, the greater a voltage of the output signal increases. Thus, the output signal from the oscillator is generally received by a high impedance circuit.
The flow meter disclosed in the publication No. 8-70926 is intricate in circuitry, and requires the wave forms of the ultrasonic wave in both the directions similar to each other. Thus, an interference of waves reflected on an inner wall of the conduit has to be considered. The conventional flow meter may be designed more difficultly under the consideration of variance at its mass production.
In the case that the two oscillators have identical properties, whichever of the oscillators is assigned to a receiving oscillator, the value (Cxc3x97Zo) of each oscillator is unchanged, and the delay times are identical to each other. This arrangement does not create an error in the measured propagation durations, which are essential for calculating the flow rate. However, in case that the two oscillators do not have identical properties, the first oscillator 2 and the second oscillator 3 have the value (Cxc3x97Zo) different from each other. Therefore, the delay time of the output signal from the receiving oscillator of one of the two oscillators is not equal to that of the receiving oscillators of other of the two oscillators. This makes the time counter 8 fail to measure the propagation durations accurately when the two oscillators are switched in the transmitting/receiving, and makes the flow-rate calculator 19 determine inaccurate flow rate.
As described above, the conventional flow meters hardly measure the flow rate accurately unless the two ultrasonic oscillators have properties identical to each other. Further, since a change in the inter-electrode capacitance caused by a temperature change is not uniform between the oscillators, as apparent from FIG. 3, a pair of oscillators having their properties substantially identical to each other has to be prepared. This preparation is a troublesome bearing process in which variations in the internal impedance and in the inter-electrode capacitance are checked while the temperature varies.
The flow meter provides accurate measurement even if employing the oscillators having their properties different from each other.